The Minoan and post-Minoan eruptions, Santorini (Greece), in the light of melt inclusions: chlorine and sulphur behaviour

Mineralogical and melt inclusion studies were carried out on plinian tephra (Pumice fall unit, Phira) ejected during the first phase of the Minoan eruption, as well as on the most basic pyroclastic ejecta emitted during the recent activity of Nea Kameni. Heating stage experiments were performed on crystallised melt inclusions. The temperature of melting (Tm) varies from 1095±10°C (Fo80–77, high Al-basaltic melts) to 995±10°C (Fo76–73, andesitic melts). A subsequent quenching and electron microprobe analysis of glass was performed in order to determine the composition of entrapped magmas. After heating experiments, the melt inclusions are parent liquids of a high Al-basaltic composition; these parent magmas have not been found as whole rocks in post-Minoan deposits. Melt inclusions, whole rocks and matrix glasses define a compositional trend from high Al-basalt to rhyolite, which attests to either mixing/mingling of cogenetic magmas or fractional crystallisation as processes controlling the chemical evolution of these magmas, and corresponds to a fraction of crystallised parent magmas of about 70%. Rhyolitic melts exhibit the highest chlorine contents (3000–3200 μg/g Cl) and the lowest sulphur contents (about 100 μg/g S). Basaltic and andesitic melts show the lowest chlorine contents (900–1000 μg/g Cl) and the highest sulphur contents (1000–600 μg/g S). This shows a difference in behaviour between chlorine and sulphur, which is probably linked to the difference in solubility. This suggests also that the present-day sulphur emission at Santorini does not originate only from differentiated magmas but also from basaltic and andesitic ones. On this assumption, the minimal chlorine and sulphur release to the atmosphere is estimated to be about 2×1010 and 2.5×1011 kg for the Minoan eruption. This shows that volcanic hazards related to volatile degassing, especially to SO2 emissions, could represent a serious risk at Santorini.